Metallic iron can be found in numerous ore deposits, some of which include goethite, hematite, magnetite and taconite. Traditionally, iron values have been collected by low-intensity magnetic separation. This process involves the use of a drum-type magnetic separator having a field intensity on the order of 3000 gauss or less. Although this process of recovery has proven extremely effective for the magnetic iron ores, it is virtually ineffective for recovering iron values from oxidized iron ores. By oxidized iron ores what is meant are those non-magnetic iron ores having an iron content of about 30 to 40 percent found in the deposits of semitaconite, oxidized taconite and hematitic-goethitic jaspers.
In the face of ever dwindling reserves of high grade magnetic iron ores, it is expected that greatly increased tonnage of these oxidized iron ores will be recovered in the foreseeable future. Thus, an efficient system of recovery becomes increasingly necessary.
The employment of traditional flotation processes for recovering oxidized iron ores prior to the 1960's resulted in the concentration of substantial tonnage of metallic iron values. However, the application of these conventional procedures presented a significant drawback, that being the excessive losses incurred during the desliming stage when significant amounts of iron oxides report to the slime fraction. In an effort to overcome this drawback, a modified flotation system was developed during the 1960's that specifically met the needs for improved flotation recovery of iron values from oxidized iron ores. This system consists of dispersing the finely ground oxidized iron ore in an aqueous medium and initially subjecting it to a selective flocculation. Following the selective flocculation stage, the system is deslimed to remove the silica-bearing fines and the flocculated residues remaining are then concentrated to final grade by a flotation of the non-ferrous siliceous material left unremoved. The essential characteristic of this system which distinguishes it from conventional flotation processes is the preconditioning of the oxidized iron ores by selective flocculation before the flotation of the siliceous gangue. This is not to say a flocculating stage is novel in the beneficiation of mineral ores. Conventional flocculants such as lime, sulfuric acid and the natural and synthetic polymeric substances have long been employed. However, in conventional applications these flocculants are employed after flotation and act on all of the dispersed solids to improve the filtration of the products or as an aid in thickening finely ground pulps. Such a use is, however, distinct from the selective flocculation conducted in the oxidized iron ore system. In selective flocculation, the flocculants are added prior to the flotation and desliming stages and are selective in their flocculating properties so as to effectuate a separation between mineral species contained in the aqueous dispersion. In the oxidized iron ore system, the selective flocculant causes the flocculation of iron containing values while leaving the non-ferrous siliceous materials in suspension.
Selective flocculants presently known in the art include tapioca flour, potato starch, natural and modified starches and polyacrylamides as taught in U.S. Pat. No. 3,292,780 issued Dec. 20, 1966 to Frommer et al. Also known is the use of acrylic acid polymers containing at least 70 mole percent of acrylic acid units as selective flocculants for recovering hematitic iron ore from siliceous materials as disclosed in U.S. Pat. No. 4,081,357 issued to Werneke et al. in 1978. Currently, the most frequently employed selective flocculant for oxidized iron ore systems is corn starch.
Although the employment of starches in the oxidized iron ore recovery as selective flocculants has proven efficient, major drawbacks continue to persist. Specifically, from an ecological vantage point, the presence of residual starches in waste water increases ts biodegradeable oxygen demand and thereby creates an environmental problem in the disposal of the waste waters. From a commercial vantage point, there are a ever increasing number of countries in which the use of reagents having a food value, such as starch, is prohibited in commercial applications.
Accordingly, there exists the need for a process of selective flocculation that will employ new selective flocculants which can both overcome the deficiencies of the conventional processes and yet maintain or improve upon selectivity and ultimate iron value recovery.